1. Field of the Invention
The present invention relates to a power transfer devide for four-wheel drive vehicles, and more particularly to a power transfer device of the type which includs a planetary gear unit arranged to be shifted between high and low-range conditions to establish torque proportioning and locked-up reduction drive modes.
2. Description of the Prior Art
In U.S. Pat. No. 4,074,591 granted to Dick on Feb. 21, 1978, there has been proposed a shift mechanism for the planetary gear unit which may be shifted to establish a high-range mode wherein the carrier acts as an input member and the sun and ring gears as output members to establish torque proportioning differential action. The shift mechanism may be further shifted to establish a low-range mode wherein the sun gear acts as an input member, the ring gear as a reaction member, and the carrier as an output member to provide locked-up reduction drive.
The shift mechanism includes a pair of shifting sleeves which are axially slidably mounted on an input shaft and one output shaft and which are linked together by a ring such that they are slidable as a unit but rotatable relative to one another. The shifting sleeves each support thereon a floating collar. The other output shaft is in the form of a hollow shaft arranged in surrounding relationship with the input shaft. In the high-range mode, one of the shifting sleeves engages the input shaft with the carrier, and its associated floating collar engages the sun gear with the other output shaft. The other sleeve engages the ring gear with the one output shaft, and its associated floating collar is disengaged. In the low-range mode, the one sleeve engages the input shaft with the sun gear, and its associated floating collar engages the carrier with the other output shaft. The other sleeve engages the carrier with the one output shaft, and its associated floating collar locks the ring gear to the housing.
In a practical embodiment of such a power transfer device as described above, it is desirable that the planetary gear unit is further arranged to establish a high-range mode under restriction of the differential action so as to enhance driving performance of the vehicle on a road covered with snow.